In a number of applications, light-weight, high strength structural members are required. For example, in motor vehicles and aircraft as well as in various devices such as home appliances and the like, a number of composite materials have been proposed in the past as structural members, such as exotic light-weight alloys. In most applications, however, mass reduction must be balanced against the cost of the product to the consumer. Thus, there is a need for providing strength without significantly increasing material and labor costs. Moreover, reinforcement techniques are needed which can be easily adapted to existing geometries of structural parts, thereby obviating any requirement for fundamental design changes.
It would be desirable to provide a structural member which may be generally considered as a wall with some form of reinforcement. While such forms of reinforcement could have numerous applications, it would be particularly desirable to provide such reinforcements for door panels, roofs, and hoods of vehicles.
A typical motor vehicle door includes an outer skin or panel of thin steel or aluminum. The outer panel is attached to structural members and, along with an inner door panel, defines a space or cavity into which a moveable window retracts. In addition to the moveable window a number of latch fixtures and window actuators are located in this door space.
Due to the large surface area of the outer panel and the thinness of the sheet metal normally used, a phenomenon as xe2x80x9coil-canningxe2x80x9d may occur. As applied to an outer door panel, oil-canning refers to the tendency of the door panel to flex from its desired position (often typically bowed) to a second, undesired position in which the outer panel is deformed or flexed inwardly toward the inner door panel. This movement may occur in response to slight pressures on the outer panel such as the weight of an individual leaning against the vehicle.
Although the oil-canning may be momentary with the panel immediately returning to its original configuration, it may be permanent and, moreover, may cause a crease to form in the panel coating. As will be understood by those in the art, creases of this kind must be removed in a body shop or the like and require that the panel be repainted.
While a number of side impact beams are known, these beams focus on the structural integrity of the motor vehicle in a crash and are not directed to the oil-canning problem. For example, a side impact beam for a vehicle door has been proposed in U.S. Pat. No. 4,978,562 which comprises an open channel-shaped metal member having a longitudinal cavity which is filled with a thermoset or thermoplastic resin-based material core. The core is disposed in the midspan of the beam. The core does not adhere to the sides of the beam. The core may include hollow glass microspheres in order to decrease density and thus weight.
Although filling large volumes of a door cavity with a foam could prevent oil-canning, such an approach would also significantly increase mass and thus weight, which, of course, is an undesirable feature. In addition, large volumes of foam contribute significantly to cost. Finally, a large foam core often creates an unwanted heat sink. And, although increasing the metal gauge of the door panel or adding localized thick metal reinforcements will increase stiffness, as the metal thickness increases weight increases.
Accordingly, there is a need for a lightweight door panel stiffener which is economical to produce and install. Such stiffeners are desirable for door panels in general even when not intended to address the oil-canning phenomenon.
The conventional method of stiffening outer automotive body panels involves placing a sheet of thermosettable polymer on the inside of a panel and then heat curing the thermosettable polymer in a paint oven. The conventional heat cured stiffener is applied as a wallpaper sheet type applique or as a sprayed-on layer. The applique is provided in a single or double layer sheet. The main layer is a thermosettable layer with or without a backing. Where a backing is used, the backing can be fiberglass cloth, metal screen, or foil. The spray-on is a single polymer layer without a backing. The polymer layer is generally 0.020 to 0.100 inches thick.
When the vehicle doors are painted, there often is a problem with paint read-through once the outer metal body panel stiffened with the thermoset polymer sheet has been heat cured. That is, the painted outer surface of the vehicle door becomes somewhat distorted as a result of the shrinkage of the thermoset polymer which takes place upon curing, thereby marring the appearance of the painted door. It would be desirable if techniques could be provided to eliminate or reduce the paint read-through problem.
Any stiffener which is to be applied to a metal body panel must be capable of adhering strongly and permanently to the body panel without delamination. This is often difficult to achieve in practice, however, as the surfaces of such body panels tend to be coated with oil or other substances which interfere with adhesion. Although the body panel surface can be degreased through the use of solvents or the like, this introduces an additional step to the production process and creates waste (i.e., used solvent or oily rags) which must then be disposed of.
A laminate useful for reinforcing or stiffening body panels is provided which includes three polymeric layers. One of the polymeric layers is a compliant layer comprised of flexible, pliable polymer which is adhered to the substrate or structural member being reinforced using a reactive thermosettable adhesive comprised of at least one polyol, at least one epoxy resin, and at least one polyisocyanate. Thermosettable adhesives of this type have been found to provide a high strength bond between the laminate and the substrate, even where the substrate surface is coated with oil or other greasy substanes. A rigid layer comprised of a thermoset resin is secured to the other side of the compliant layer. The rigid layer improves the resistance of the substance against bending or flexing. A carrier, such as a foil or fiberglass backing, may be secured to the outer surface of the rigid layer. The laminates of the present invention have the further advantage of dampening acoustic and other vibrations, thereby reducing the level of noise in the interior of a vehicle having panels reinforced with such laminates.